1. Field of the Invention
The present invention relates to a high temperature oxidation resistant carbonaceous molding for use in an oxidizing atmosphere and its manufacturing method, and more particularly to a high temperature oxidation resistant carbonaceous molding suitably used under the environments where high and low temperatures are repeated, and its manufacturing method.
2. Description of the Related Art
A carbon material has a greater strength than a metal material in a non-oxidizing atmosphere at high temperatures, but easily burns in an oxidizing atmosphere such as the atmospheric air, as well known. Accordingly, if the surface of carbon material is covered with a heat resistant material to shut off oxygen, the carbon material brings out its proper strength in the oxidizing atmosphere at high temperatures.
Conventionally, a carbonaceous molding has been known in which the oxidation resistance is increased by covering the surface of carbon material with a vitreous material to shut off oxygen (e.g., Published Japanese Patent Application JP-A-9-301786 page 4, right column, 36th line to page 6, right column, 9th line). However, since a vitreous material molten at high temperature has a poor wettability to carbon material, the surface of this carbonaceous material is not uniformly covered with vitreous material. Particularly, if this carbonaceous molding is employed under the environments where high and low temperatures are repeated, the vitreous material is detached from the carbon material, so that the carbon material is exposed to the oxidizing atmosphere.
Also, conventionally, there is well known a method for forming a chromium diffusion layer on the surface of a workpiece through the chromising treatment (e.g., Published Japanese Patent Application JP-A-4-280955 page3, left column, 35th to 39th line). This chromising treatment involves contacting the workpiece with, for example, a metal penetrant containing metal chromium powder, halide and a sintering inhibitor, and heating them in a ventilating current of hydrogen gas to diffuse chromium into the workpiece. In this chromising treatment, a vapor of chromium halide produced by reaction between metal chromium and halide is reduced on the surface of the workpiece by hydrogen gas and becomes fine metal chromium of high purity. And metal chromium deposited on the surface of the workpiece is diffused into the workpiece, so that a chromium diffusion layer is formed on the surface of the workpiece.
If the chromising treatment is applied to carbon material, the chromium diffusion layer superior in the heat resistance containing chromium carbide (Cr23C6) produced by bonding between carbon and diffused metal chromium is formed on the surface of carbon material. This chromium diffusion layer has a more excellent wettability to vitreous material at high temperatures than carbon material, whereby the vitreous material layer is uniformly formed on the surface of the chromium diffusion layer. Accordingly, if the chromium diffusion layer and the vitreous material layer are employed to shut off oxygen, it may be possible to add the high oxidation resistance to carbon material.
However, if the carbon material is subjected to the conventional chromising treatment, the carbon material and hydrogen gas are reacted to produce hydrocarbon, so that the carbon material of a base material is consumed. As a result, the chromium diffusion layer formed on the surface of carbon material is likely to be peeled. Accordingly, the carbonaceous molding with the vitreous material layer covered on the chromium diffusion layer is not suit for use under the environments where high and lower temperatures are repeated.